In the operation of lock stitch type sewing machines, it is necessary to provide not only a primary thread supply to the sewing needle, but also a secondary supply of thread to a bobbin, located on the opposite side of the work from the needle. In the sewing operation, the needle penetrates the work, carrying a length of thread from the main supply. Before the needle is retracted, the thread is engaged from below the work, and a loop of the thread is passed around the bobbin in order to engage the bobbin thread and form the desired lock stitch. Because of the requirement that the loop of the needle thread be passed completely around the bobbin, the bobbin and its thread supply are necessarily quite small. In a high speed industrial sewing operation, the bobbin supply normally is exhausted at frequent intervals. Under normal circumstances, this results in at least the inconvenience of stopping the sewing operation while a new bobbin in inserted in the bobbin case and the empty bobbin is removed for refilling. Especially undesirable, moreover, is the prospect of exhausting the bobbin supply during the sewing of a workpiece. This necessitates restarting the sewing operation in the middle of the workpiece. For many products, that would destroy the value of the goods (e.g., where the stitching is highly visible, as in the lapel of a jacket, for example). To avoid such an eventuality, it is necessary to reload the machine with a full bobbin considerably in advance of exhaustion of the supply of the old, so that bobbins are changed oftener than necessary and there is a substantial wastage of bobbin thread.
Pursuant to the teachings of the Rovin U.S. Pat. No. 3,509,840 previously referred to and incorporated herein by reference, the bobbin spool is loaded with a predetermined length of thread, calculated to be just enough to complete a given work unit, consisting of a single workpiece, if relatively large, or a succession of workpieces, if relatively small. At the end of the sewing of the predetermined work unit, the bobbin supply is exhausted, with only a minimum "tail" being provided to accommodate normal operating tolerances. In a typical case, a couple of inches of such tail may be adequate. After the sewing of the work unit has been completed, and while a new work unit is being put into place, a loading mechanism moves into place with respect to the bobbin spool, rotating the spool at extremely high speed through an air-driven turbine arrangement of which the spool itself forms the moving part, and advancing a thread onto the spool, to be picked up and driven by the spool hub, by means of a hollow loading tube through which thread is advanced by the flow of air. After a predetermined length of thread is wound on to the bobbin spool, a process that is accomplished in about one second of time or less, the loading mechanism retracts, the thread is guided under the bobbin spring, and the thread is then severed from the bobbin supply source, permitting the next sewing cycle to commence.
In accordance with the present invention, a mechanism is provided to perform the functions described above, which is both highly compact and relatively simplified in its construction, enabling it to be produced on an economical basis and to be mounted as an attachment on various lock stitch type sewing machines of commercially available design. The mechanism of the present invention incorporates a short stroke, double acting air cylinder which, in its retracting and extending strokes, serves to execute a multiplicity of functions involved in the reloading of the bobbin spool. In timed sequence, these include the following primary functions: (1) Initiation of turbine air to rotate the bobbin spool at high speed; (2) direction of air through the thread-loading tube; (3) advancement of the thread-loading tube into loading position with respect to the bobbin spool; (4) release of the thread for injection onto the bobbin spool; (5) retraction of the loading tube; (6) manipulation of the thread under the bobbin case tension spring (for controlling bobbin thread tension during sewing); (7) return of the thread manipulating mechanism; (8) severing of the thread from the supply source; (9) clamping the thread within the loading mechanism, in preparation for a further cycle. The relatively complex series of operations thus described, is accomplished in a single reciprocation of a fluid actuator, by means of a compact, relatively easily manufactured mechanism, which is of sufficiently compact form to be easily received underneath the sewing plate of a conventional lock stitch type sewing machine.
In some commercially available lock stitch type sewing machines, the bobbin is mounted for rotation about a vertical axis; in others, it is mounted for rotation about a horizontal axis. The mechanism of the invention is readily adaptable to either arrangement, with minimum alterations in the mounting arrangements.
In accordance with another significant aspect of the invention, a novel and improved, economical control arrangement is provided for effecting precise measurement of the thread, as it is being fed on to the bobbin spool, and for terminating the loading operation after a predetermined length of thread has been wound on the spool. The control system is readily adjustable to provide for variation in the length of the bobbin thread to accommodate different working units. The control system of the invention includes means for generating an electrical pulse in accordance with the advance of a predetermined unit length of thread. Control over the loading operation is not accomplished, however, by merely counting the pulses just generated, as systems for this purpose would be unduly costly. The system of the invention, on the other hand, includes highly simplified, inexpensive yet reliable solid-state circuit arrangements which shape and form the pulsed electrical signals to control pulses of uniform size and shape, independent of the fact that the pulses as originally generated vary both as to amplitude and as to duration. The uniformly shaped pulses are then accumulated in an integrator circuit, and the output of the integrator circuit is summed with an adjustable control voltage of opposite polarity, the control voltage being set in accordance with the desired amount of thread to be loaded on to the spool. The output of the voltage summing junction is fed to a bi-stable comparator circuit, including a high-gain amplifier. As soon as the integrated voltage even slightly exceeds the control voltage, the bi-stable comparator circuit flips to the opposite polarity and executes control functions to terminate the loading operation. Among these control functions are instant engagement of the thread by a braking device and initiation of the reverse stroke of the air actuator.
The circuit arrangement of the invntion provides a high degree of precision for the purposes intended, yet may be manufactured with great economy, so that the entire system, including the mechanical mechanisms in the related controls, may be marketed on an economically attractive basis.